Wednesday, January 12, 2011

Is the Siberian Express Coming?

Would you believe that there was fresh snow in 49 states today?  Only Florida lacks snow.  Another sign of interesting times?  Raleigh, NC is also on a trajectory that could place this winter in the coldest top 5!

I find it very interesting that some computer model runs are trying to forecast a positive Arctic Oscillation yet have consistently not been negative enough!  For nearly enough the actual results (black line) has been at or below the range of forecasts.
Source: NOAA Climate Prediction Center
The -AO occurs when Upper Level High Pressure dominates the North Pole thereby reversing the Polar Easterlies and displacing the coldest air to the south.  This feature does not guarantee that we on the Eastern Seaboard will get targeted by the cold because a number of other features like this winter's -NAO to route the air southeast.

Similar to 2009-10 warming in the Stratosphere is maintaining Arctic High Pressure and now that feature has gone to further extremes. The 2 small blue areas consistently preceded warming trends by near 2 weeks.  Notice how the warming now has gone all the way to the top:
Source: NOAA Climate Prediction Center
It has been near the top for several days which means that we should be aware that the coldest air so far this winter is coming south.  Projections of NW US High Pressure (called the +PNA) suggest that we in the East are the primary targets!  Mercifully the NAO is slated to be neutral or even slightly positive for the first time since mid November.  Those projections have been accurate throughout this season.  This means that the cold, while serious is likely to lack a mechanism to hold it in place for extended periods.

This Northern Hemisphere projection based on the European Model (ECMWF) depicts a scary scenario which a mountain of High Pressure in Siberia pushes a strong Arctic Vortex into North America.  Off the West Coast that High is slated to build and move inland giving the cold only one direction to go ... Southeast!   On a hopeful note: this kind of pattern is not stable and will not be long lasting.

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